Alzheimer's disease is the most common form of dementia in the aging population. In Alzheimer's disease, an important biochemical characteristic is the extracellular accumulation of amyloid β (Aβ), especially Aβ1-42, to form the insoluble amyloid plaques in brain tissue of Alzheimer's patients. Aβ is produced by proteolytic cleavages of amyloid precursor protein (APP) (Masters & Selkoe (2012) Cold Spring Harb. Perspect. Med. A006262). Recent evidence indicates that the size of the amyloid plaques, which mainly consist of aggregates of the fibrillar form of Aβ, does not correlate well with the degree of neurodegeneration or the severity of dementia in Alzheimer's disease (Lublin and Gandy. 2010. Mt. Sinai J. Med. Transl. Pers. Med. 77:43-49). Instead, the oligomeric forms of Aβ, which are intermediate forms between the monomeric and the fibrillar forms, have been suggested to be the most toxic molecular species that cause synaptic loss (Koffie et al. (2009) Proc. Natl. Acad. Sci. USA 106:4012-4017; Lesne et al. (2006) Nature 440:352-357; Martins et al. (2008) EMBO J. 27:224-233; Shankar et al. (2008) Nat. Med. 14:837-842).
Studies have shown that cholesterol content in cells can affect the production of Aβ, in part by the ability of cholesterol to modulate the enzyme activities of various secretases in cell membranes (Wolozin (2004) Neuron 41:7-10). Cholesterol metabolism has also been implicated in the pathogenesis of Alzheimer's disease in other manners (Jiang et al. (2008) Neuron 58:681-693; Wellington (2004) Clin. Genet. 66:1-16; Hartmann (2001) Trends Neurosci. 24:S45-48).
In the brain, cholesterol is derived from endogenous biosynthesis (Dietschy & Turley (2004) J. Lipid Res. 45:1375-1397). The transcription factor SREBP2 controls the expression of enzymes involved in cholesterol biosynthesis, including the rate-limiting enzyme HMG-CoA reductase (HMGR) (Goldstein et al. (2006) Cell 124:35-46). Other transcription factors, including liver X receptors (LXRs), control the expression of proteins which function in cholesterol transport (Repa & Mangelsdorf (2000) Annu. Rev. Cell Dev. Biol. 16:459-481; Beaven & Tontonoz. (2006) Annu. Rev. Med. 57:313-329), including apoE, ABCA1, and others (Wang, et al. (2008) FASEB J. 22:1073-1082; Tarr & Edwards (2008) J. Lipid Res. 49:169-182). In the brain, cholesterol can be enzymatically converted by a brain-specific enzyme, 24-hydroxylase (CYP46A1) (Russell et al. (2009) Annu. Rev. Biochem. 78:1017-1040), to an oxysterol called 24S-hydroxycholesterol (24SOH); the concentration of 24SOH far exceeds those of other oxysterols in the brain (Lutjohann et al. (1996) Proc. Natl. Acad. Sci. USA 93:9799-9804; Bjorkhem. (2006) J. Intern. Med. 260:493-508; Karu et al. (2007) J. Lipid Res. 48:976-987). Various oxysterols, including 24SOH, can down-regulate sterol synthesis in intact cells and in vitro (Song et al. (2005) Cell Metab. 1:179-189; Wang et al. (2008) J. Proteome Res. 7:1606-1614). When provided to neurons, 24SOH decreases the secretion of Aβ (Brown et al. (2004) J. Biol. Chem. 279:34674-34681). However, whether 24SOH or other oxysterols can act in similar fashion(s) in vivo remains to be demonstrated. 24SOH levels have been shown to be decreased in brain samples from Alzheimer's disease patients (Heverin et al. (2004) J. Lipid Res. 45:186-193), suggesting a relationship between 24SOH and Alzheimer's disease.
Acyl-CoA: Cholesterol Acyltransferase (ACAT) converts free cholesterol to cholesterol ester, and is one of the key enzymes in cellular cholesterol metabolism. Two ACAT genes have been identified which encode two different enzymes, ACAT1 and ACAT2 (also known as SOAT1 and SOAT2). ACAT1 and ACAT2 have different tissue expression patterns (Chang et al. (2009) Am. J. Physiol. Endocrinol. Metab. 297:E1-E9). ACAT1 is a resident enzyme in the endoplasmic reticulum and is ubiquitously expressed in all tissues examined, while ACAT2 is expressed mainly in the intestines and liver (Chang et al. (2009) Am. J. Physiol. Endocrinol. Metab. 297:E1-E9). Early studies showed that in cells expressing human APP, inhibiting ACAT activity significantly reduced the amount of Aβ secreted into growth medium (Puglielli et al. (2001) Nat. Cell Biol. 3:905-912). Alzheimer's disease-like pathology has been demonstrated in the brains of transgenic mice expressing human APP(751) containing the London (V717I) and Swedish (K670M/N671L) mutations (Hutter-Paier, et al. (2004) Neuron. 44(2):227-38). Two months of treatment with CP-113,818, a non-selective ACAT inhibitor, was shown to reduce the accumulation of amyloid plaques by 88%-99% and membrane/insoluble Amyloid β levels by 83%-96%, while also decreasing brain cholesteryl-esters by 86%. Additionally, soluble Amyloid β(42) was reduced by 34% in brain homogenates. Spatial learning was slightly improved and correlated with decreased Amyloid β levels. In nontransgenic littermates, CP-113,818 also reduced ectodomain shedding of endogenous APP in the brain. A 50% decrease in ACAT1 expression has also been shown to reduce cholesteryl ester levels by 22%, reduce proteolytic processing of APP, and decrease Amyloid β secretion by 40% (Huttunen et al. (2007) FEBS Lett. 581(8):1688-92) in an in vitro neuronal cell line.
Macroautophagy, or autophagy, is a conserved lysosomal degradation process that begins with sequestration of certain cytoplasmic content with a double-membrane structure, followed by formation of an autophagosome (Mizushima. (2007) Genes Dev. 21:2861-2873). Autophagosomes fuse with lysosomes to degrade sequestered cytoplasmic contents, including denatured and/or aggregation-prone proteins/peptides, such as Aβ (Mizushima et al. (2008) Nature 451:1069-1075). Autophagosome formation can be induced by inhibition of the mammalian target of rapamycin (mTOR) (Mizushima, (2007) Genes Dev. 21:2861-2873). Inhibition of mTOR signaling also up-regulates lysosome biogenesis and leads to efficient autophagosome-lysosome fusions (Zhou et al. (2013) Cell Res. 23:508-523). The transcription factor EB (TFEB), a newly discovered master regulator of lysosomal protein biogenesis (Sardiello et al. (2009) Science 325:473-477), coordinates these two processes by activating the autophagic machinery and by increasing the expression of lysosome-specific genes (Settembre et al. (2011) Science 332:1429-1433; Settembre et al. (2012) EMBO J. 31:1095-1108; Zhou et al. (2013) Cell Res. 23:508-523). In mouse models of Alzheimer's disease, studies have shown that blocking mTOR by rapamycin administration increases autophagy in the brain, reduces Aβ1-42 levels, and rescues cognitive deficits (Caccamo et al. (2010) J. Biol. Chem. 285:13107-13120; Spillman et al. (2010) PLoS ONE 5:e9979).